Condensation polymers such as thermoplastic polyesters, polycarbonates, and polyamides have many desirable physical and chemical attributes that make them useful for a wide variety of molded, fiber, and film applications. However, for specific applications, these polymers also exhibit limitations that must be minimized or eliminated. To overcome these limitations, polymers are frequently made containing one or more additives or comonomers depending upon the desired end use of the polymer. For example, in the case of polyester polymers, it is common to incorporate one or more ultra-violet absorbers, particles for improving the reheat of bottle preforms made from the polyester polymer, toners or pigments or colored particles, acetaldehyde scavengers or inhibitors, catalyst deactivators or stabilizers, oxygen barrier material, friction reducing aids, crystallization aids, impact modifiers, and so forth.
One of the most common thermoplastic polyester polymers is polyethylene terephthalate (PET). PET polymer is used extensively in the packaging industry, especially in the production of bottles for carbonated and non-carbonated beverages. In the carbonated beverage industry, concerns include the rate of carbon dioxide escape from the container, taste deterioration of the contents due to degradation by light, and extraction of additives added either during melt polymerization or subsequent melt processing required to fabricate the container. To overcome these problems, PET resins are often modified by incorporating unique comonomers into the polymer backbone thus producing a wide variety of PET copolyesters.
Several techniques have been employed to produce PET copolyesters. In conventional polyester manufacturing, copolyesters are typically produced by two different routes: ester exchange plus polycondensation (the DMT process) or direct esterification plus polycondensation (the direct esterification process.) In the older ester exchange process, or DMT process, dimethyl terephthalate (DMT), ethylene glycol (EG), and the modifying comonomers are typically combined at the beginning of the manufacturing process in the paste tank or first esterification reactor; the modifying comonomer can be added as either a diacid, a dialkyl ester derivative of the acid, or a diol. In the presence of a catalyst at atmospheric pressure and at a temperature from about 180° C. to 230° C., these components undergo ester interchange to yield the intermediate bis(hydroxyethyl ester) substituted monomer of the acids and methanol.
The reaction is reversible and is carried to completion by removing the methanol. After completion of the ester exchange, a stabilizer may then be added to deactivate the ester exchange catalyst and a polycondensation catalyst is then added. The intermediate monomers are then polymerized by a polycondensation reaction, where the temperature is raised to about 265° C. to about 305° C. and the pressure is reduced to below 2 mm of mercury vacuum in the presence of a suitable polymerization catalyst (e.g. antimony). From this reaction, polyethylene terephthalate copolymer and ethylene glycol are formed. Because the reaction is reversible, the glycol is removed as it is evolved, thus forcing the reaction toward the formation of the polyester.
The second method, or direct ester exchange process, is a well known variation of the DMT process and utilizes purified terephthalic acid (PTA) instead of DMT. In the first step, PTA is combined with ethylene glycol (EG) and either diacids or diols of the modifying comonomers. Typically reacted without any catalyst and at a pressure from about 5 psia to 85 psia and at a temperature from about 155° C. to 305° C., these components undergo direct esterification to yield bis(hydroxyalkyl) substituted intermediate monomers of the acids and water. After the completion of the direct esterification, the intermediate monomers are then polymerized by the polycondensation reaction as described in the ester interchange process. That is, the intermediate monomers are then polymerized by raising the temperature to about 265° C. to about 305° C. and the pressure is reduced to below 2 mm of mercury vacuum in the presence of a suitable polymerization catalyst (e.g. antimony). From this reaction, polyethylene terephthalate copolymer and ethylene glycol are formed. Because the reaction is reversible, the glycol is removed as it is evolved, thus forcing the reaction toward the formation of the polyester.
With the increased availability of purified terephthalic acid, the newer direct esterification offers many advantages including conversion from a batch process to a continuous process. Continuous processes are cost effective to operate when relatively large amounts of polyester or copolymer are required. However, other problems occur relating to the use of the continuous process to make copolyester when relatively small amounts of copolyester are required and/or a family of copolyesters with varying comonomer content is desired. In particular, a residence time on the order of 4-to-12 hours is typical for a continuous polymerization process; therefore, any changes in polymer compositions will generate significant amounts of off-class material. Problems associated with off-class are further exacerbated at higher production rates on larger scale manufacturing lines making such equipment ill-suited for small-scale production of a diverse family of modified thermoplastics resins containing varying compositions.
Several post-polymerization processes have also been utilized to produce PET copolyesters. One approach has been to melt blend a PET base polymer with a second, condensation type polymer using an extruder or reaction vessel, allowing the two polymers to undergo transesterification, thereby producing a random copolymer. This process exhibits several shortcomings. First, extended reaction times up to ⅓ to two hours are required to achieve melt randomization thereby leading to thermal degradation of the polymers and concurrent loss in physical properties, generation of color, and production of other undesirable degradation products such as acetaldehyde. Second, additional equipment either in the form of large-scale, heated vessels or extruders are required to provide the extended melt residence time. Third, additional catalysts can be incorporated to facilitate tranesterification thereby reducing the melt residence time; however the additional catalysts will negatively impact both the color and thermal stability of the resultant homogenized copolymer. Lastly and most important, this process does not capture the economic advantages associated with large-scale continuous polyester manufacturing processes.
Another post-polymerization process to prepare copolyesters comprises merging two reactor melt streams together wherein a melt stream of highly modified copolyester is feed into the discharge line of continuously polymerized polyester of low or no modification and subjecting the resins to static mixing and dynamic mixing in the discharge line. A variant of this method involves withdrawing a side steam from the discharge line of the continuously produced PET base resin, sending the side stream to an kneading extruder, introducing the highly-modified copolyester into the kneading extruder, returning the modified side stream to the discharge line of the continuously produced PET base resin, then subjecting the resins to static mixing and dynamic mixing in the discharge line. Although the combination of static and dynamic mixing of these approaches likely improves the distributive mixing of these two components, it does not provide sufficient time to enable reactive randomization via transesterification of the two polyesters. Furthermore, the addition of static and dynamic mixing units in the transport lines of a large-scale, continuous polymerization have the disadvantage in that they require extensive cleaning between product transitions, limiting their utility for agile production of a portfolio polyester products.
In light of the above, there exists a need for a method to effect more rapid compositional changes during product transitions with generation of less off-class material in the production of copolyester resins by melt phase polymerization, particularly in relation to large-scale, continuous plants designed to produce such polyesters. Thus, there is a desire to provide a method for adding a modifying polymer to a polymer melt stream in a manner which allows time for good mixing and chemical equilibration.